The present exemplary embodiment relates to an image processing system for decompressing image data.
With the expansion of the Internet as well as the explosive growth of digital storage devices, the use of compression techniques for storing and transporting multimedia content is becoming increasingly important. One multimedia compression protocol is JPEG (Joint Photographic Experts Group), which is used for storing still photographs or images. The original JPEG standard was created in the late 1980s and has become widely used on the Internet as well as in digital imaging equipment. In 1997, a call for proposals for the next generation JPEG standard, called JPEG 2000, was issued by the International Organization for Standardization (ISO), under whose guidance the original JPEG standard was promulgated. The first part of the draft standard (ISO/IEC 15444-1: JPEG 2000 image coding system) was released in 2000.
JPEG 2000 (also referred to as “J2K”) supports both lossy and lossless compression of single-component (e.g., greyscale) and multi-component (e.g., color) imagery. In addition to this basic compression functionality, other features are provided, including 1) progressive recovery of an image by fidelity or resolution; 2) region-of-interest coding, whereby different parts of an image may be coded with differing fidelity or resolution; 3) random access to specific regions of an image without needing to decode the entire codestream; 4) flexible file format; and 5) good error resilience. Due to its improved coding performance and many attractive features, there is a very large potential application base for JPEG 2000. Some potential application areas include: image archiving, web browsing, document imaging, digital photography, medical imagery, and remote sensing.
At the core of the JPEG 2000 structure is a new wavelet based compression methodology that provides for a number of benefits over the Discrete Cosine Transformation (DCT) compression method, which was used in the original JPEG format. The DCT compresses an image into 8x8 blocks and places them consecutively in the file. In this compression process, the blocks are compressed individually, without reference to the adjoining blocks. This results in “blockiness” associated with compressed JPEG files. With high levels of compression, only the most important information is used to convey the essentials of the image. Thus, much of the subtlety that makes for a pleasing, continuous image is lost.
Nonetheless, during the standard JPEG 2000 compression process it has been found that undesirable oscillations, sometimes called “ringing,” may be introduced in the compressed images. Ringing in JPEG 2000 images may be due, in part, to course quantization of high frequency wavelet coefficients. The ringing artifacts may be particularly severe in document images, where sharp edges are present in text and graphics are more likely to occur. Thus, quantization of wavelets is a major source of information loss in JPEG 2000 compression. As quantization is an M to one mapping (i.e., the same code can be generated by many different source images), it is difficult to guarantee the retrieval of the original input data.
Thus, there is a need for a JPEG 2000 decompression method and system that reduces ringing artifacts without introducing new artifacts.